An autonomous approach must make it possible to fly the aircraft under instrument-based meteorological flight conditions of IMC (“Instrument Meteorological Conditions”) type, up to a decision height, for example 200 feet (about 60 meters), which corresponds to the decision height for a precision approach of ILS (“Instrument Landing System”) type, without using navigation means situated on the ground. During an autonomous approach such as this, the positioning means used by the aircraft are therefore exclusively onboard means, and in particular a satellite-based positioning system, for example of GPS (“Global Positioning System”) type, an inertial platform, and barometric sensors. The approach can also be carried out without an air space controller. At said decision height, the crew of the aircraft must visually acquire the runway. If such is the case, they finish the landing in visual mode with the aid of manual guidance. On the other hand, if at this decision height, the crew do not see the runway, they must perform a go-around so as to make the aircraft climb back up to a predetermined safety altitude. The approach trajectory used to carry out an autonomous approach such as this is created before the flight and described on a paper chart which is at the disposal of the crew in the course of the flight. Accordingly, the configuration of the terrain around the landing runway has been analyzed and margins in relation to obstacles have been defined. This approach trajectory is also saved in a database of the aircraft piloting system.
Document FR-2 852 686 discloses a system for piloting an aircraft intended to pilot the aircraft during a non-precision approach. This piloting system is used especially in the civil sector. A standard piloting system such as this, which therefore relates to a non-precision approach, presents a major objective common to an autonomous approach such as considered in the present invention, namely that said piloting system must be autonomous in relation to the navigation means situated on the ground and must thus be able to carry out the approach at any location on the terrestrial globe.
A standard piloting system such as this, intended for a non-precision approach, generally comprises:                a flight management system of FMS type (“Flight Management System”), which determines the actual position of the aircraft, doing so:        in the nominal case, by performing a GPS (“Global Positioning System”) information hybridization of a multi-mode landing aid receiver of MMR (“Multi Mode Receiver”) type with inertial information arising from an inertial reference unit; and        in degraded cases, by performing a hybridization of the inertial information with radionavigation information;        a multi-mode landing aid receiver, of MMR type, which receives, in particular from said flight management system, the actual position of the aircraft and information making it possible to characterize a virtual approach axis, and which on the basis of this information determines lateral and vertical angular deviations between the actual position of the aircraft and the position that it would have if it were on said virtual approach axis;        a flight guidance computer of FG (“Flight Guidance”) type which calculates, on the basis of the angular deviations received from said multi-mode landing aid receiver, and on the basis of the distance with respect to the axis of the landing runway, linear deviations and which uses these linear deviations to calculate, with the aid of at least one standard piloting law, aircraft guidance setpoints; and        at least one piloting aid device, for example an automatic pilot or a flight director, which allows the guidance of the aircraft up to the landing thereof, in accordance with said flight setpoints, doing so either automatically (automatic pilot), or manually (flight director).        
It will be noted however that the objectives of an autonomous approach, in particular in the military sector, are more ambitious than those of a non-precision approach of aforesaid type in the civil sector. In particular, the decision heights envisaged are a minimum of 200 feet (about 60 meters) for an autonomous approach, while they are a minimum of 250 feet (about 75 meters) for a non-precision approach. Consequently, the safety cone in which the aircraft must remain in order to perform the approach is smaller in an autonomous approach than in a non-precision approach. Moreover, in an autonomous approach, the absence of any air traffic controller removes a surveillance means, as compared with the situation existing for a non-precision approach in the civil sector.
Thus, the standard piloting system of aforesaid type, which is very appropriate for piloting an aircraft during a non-precision approach, makes it possible to guarantee:                neither the upkeep of the performance demanded for an autonomous approach, the most constraining performance being the integrity and precision of the vertical positioning of the aircraft, as well as the flexibility of the automatic pilot making it possible to capture segments of short length and to precisely maintain small and large slopes;        nor compliance with the critical nature of certain parameters (in particular the virtual approach axis according to which the aircraft will be guided, as well as the three-dimensional position of said aircraft) used by the piloting system.        